Orbital Rashba effects and light-induced Orbital Current in Teraherz Emission Experiments.

Orbitronics is based on the use of orbital currents as information carriers. These may be generated by the application of an electric field, and inversely, by reciprocity, they could be converted back to charge or even spin currents. In this presentation, I will start my presentation by describing experiments showing the strong enhancement of the current-induced torques on the Co layer of a Pt/Co bilayer by the addition of an Al layer on Co (Al protected from oxidation). The enhancement is predominant for the FL torque, up to factor of 9. The interpretation [2] comes from ab-initio calculations showing large Co orbital moments in the interfacial Co layer with a helical texture similar to the spin texture on the surface of topological insulators. The calculation of the resulting torques leads to a good agreement between the calculated and enhanced experimental torques.

In a second part, we will demonstrate that orbital currents can also be generated by femtosecond light pulses and optical absorption on thin Ni films. In multilayers associating Ni with oxides and nonmagnetic metals such as Cu, we detect the generated orbital currents by their conversion into charge currents varying in time in the subpicosecond scale and thus resulting in a terahertz emission as provided by oscillating dipoles. We show that the orbital currents extraordinarily predominate the light-induced spin currents in Ni-based systems, whereas, unlike, only spin currents can be detected with CoFeB-based similar systems and heterostructures. In addition, the analysis of the time delays of the terahertz pulses leads to relevant information on the group velocity and subsequent propagation of orbital carriers. Our finding of light-induced orbital currents and our observation of their conversion into charge currents opens new avenues in orbitronics, including the development of novel type of orbitronic terahertz devices